It is expected that a relay station (RS) will be widely used in a wireless communication system for next generation. Hereinafter, the relay station (RS) will be described in brief.
A standardization project newly titled multi-hop relay is being in progress in the IEEE (Institute of Electrical and Electronics Engineers) 802.16 of 2006 since the standard IEEE 802.16-2004 based on fixed subscriber mobile stations and the standard IEEE 802.16e-2005 for providing mobility of a subscriber mobile station.
This standardization project handled by a task group j (IEEE 802.16j) of the IEEE 802.16 has discussed usage models, terminologies, and technical requirements in the second conference in July, 2006 since the first conference in May, 2005. Hereinafter, the IEEE 802.16 task group j will be abbreviated as “802.16j”.
The relay station to be described hereinafter will substantially be the same as a relay station considered by a 3GPP IMT-A (LTE-A) system. Also, a relay station that performs same or similar functions in various wireless access systems will be used similarly to the relay station described herein.
Project authorization request (PAR) of the 802.16j has two standardization tasks, i.e., coverage extension and throughput enhancement.
A network that uses a relation station includes a base station (BS), a relay station (RS), and a mobile station (MS). The mobile station can receive a radio signal through the relay station even outside a cell zone of the base station. Also, a mobile station located within the cell zone of the base station can establish a path of high quality, which has an adaptive modulation and coding (AMC) scheme of high level through the relay station. Accordingly, a user can obtain enhancement effect of overall system capacity through the same radio resource.
The standard to be made by the 802.16j project has predetermined requirements. For example, a mobile station based on the existing 802.16-2004 and 802.16e-2005 standards should perform communication with the relay station without additional function. Accordingly, an application range of the relay station can be limited to the existing system in such a manner that some functions for controlling the relay station are added to the relay station and the existing base station. It is expected that the standard of the relay station will serve as a main factor of the standardization
The relay station could be regarded as a subscriber mobile station that performs operations of a physical layer and a medium access control layer. Although the relay station is mainly controlled by the base station, it may have a predetermined control function if necessary. Various relay stations will be considered as usage models which are currently discussed, and their examples include a fixed relay station, a mobile relay station for providing a temporary service to a specific zone, and a relay station that can be built in a car or subway.
Main technical issues which will be discussed later are as follows:
1. a procedure of identifying relay stations that exist in an zone of a base station and acquiring and maintaining information of a topology with the relay stations;
2. definition of a structure of a physical transport frame between a relay station and a mobile station having backward compatibility with the existing IEEE 802.16 system;
3. a signal procedure for providing mobility between relay stations or between a relay station and a base station; and
4. a network entry procedure of a relay station to a base station and a network entry procedure of a mobile station through a relay station.
Hereinafter, a structure of a general frame used in a wireless access system will be described.
FIG. 1 is a diagram illustrating a frame structure used in a broadband wireless access system (for example, IEEE 802.16).
Referring to FIG. 1, a horizontal axis of a frame is a time unit and represents orthogonal frequency division multiple access (OFDMA) symbols, and its vertical axis is a frequency unit and represents physical numbers of a subchannel. In FIG. 1, one frame is divided into data sequence channels for a certain time period by physical characteristics. In other words, one subframe includes a downlink subframe and an uplink subframe.
At this time, the downlink subframe may include a preamble, a frame control header (FCH), a downlink map (DL-MAP), an uplink map (UL-MAP), and one or more data bursts. Also, the uplink subframe can include one or more uplink data bursts and a ranging subchannel.
In FIG. 1, the preamble is specific sequence data located in a first symbol of each frame and is used so that the mobile station synchronizes with the base station or estimates a channel. The FCH is used to provide channel allocation information related to the DL-MAP and information of channel code. The DL-MAP/UL-MAP are medium access control (MAC) messages used by the downlink/uplink to indicate channel resource allocation to the mobile station. Also, the data burst represents a unit of data intended to be transmitted from the base station to the mobile station or from the mobile station to the base station.
A downlink channel descriptor (DCD) that can be used in FIG. 1 represents a MAC message for indicating physical characteristics of a downlink channel, and an uplink channel descriptor (UCD) represents a MAC message for indicating physical characteristics of an uplink channel.
In case of the downlink, referring to FIG. 1, the mobile station synchronizes with the base station by detecting the preamble transmitted from the base station. Afterwards, the mobile station can decode the downlink map using information acquired from the frame control header (FCH). The base station can transmit scheduling information for downlink or uplink resource allocation to the mobile station per frame (for example, 5 ms) using a downlink or uplink map (DL-MAP/UL-MAP) message.